Quality wise I think ATI skimps on their fans and they die out relatively early (2-3 years). Nvidia is also (for the most part) better at writing drivers (imo/experience).

That being said, i'm running a 4890 right now. Got it for 239 w/ $20 MIR and is a good overclocker. The difference really changes from year to year. I try to alternate between companies... whenever one of them lags behind we tend to get price raped by the industry leader.